Nihal can do it in 15 days.

If they finish in 18 days, it means that Nihal finishes the job (he works every 2nd and 3rd day of a 3-day shift, 18/3 is last day of 6th shift). During the 18 days, Jatin has worked 6 days and Nihal 12 days. They could also had finished the work if they both worked 10 days. That means that 2 days of Nihal (12-10) equals 4 days of Jatin (10-6).

The 10 days of Jatin could be done by Nihal in only 5 days. Add the original 10 days results in 15 days.